This invention relates to refrigeration systems, and especially closed and sealed refrigeration systems which rely on circulating a refrigerant through steps of compression, condensation, and expansion, whereby heat can be absorbed from a medium to be cooled, and subsequently rejected to a heat sink. By "closed and sealed," we mean that the refrigerant system, during routine operation, is closed and sealed against addition or removal of the working fluids, namely the refrigerant and lubricating oil. This corresponds with the meaning of "closed and sealed" as generally accepted in the refrigeration art.
It is known to assemble a small refrigeration system, such as for air conditioning a home, in a single supporting framework. These small systems can be picked up as unitary systems and moved about at will. Such systems typically use conventional chlorofluorocarbon refrigerants and are typically limited in cooling capacity to 100,000 Btu per hour or less.
It is also known to assemble a larger capacity refrigeration system at the use site whereby one or more of the various system elements such as the compressor or one or more of the heat exchangers are mounted separately to a building or the like at the use site.
It is further known to use ammonia as the refrigerant gas, and wherein at least part of the heat absorbed by the ammonia refrigerant is removed from the refrigeration system by a stream of cooling liquid such as water or the like.
Especially with respect to refrigeration systems which use ammonia as the refrigerant, lubricating oil may become intermingled with the refrigerant in the compressor as a secondary effect of injecting the lubricating oil into the compressing cavity as a means of lubricating the compressor. The material leaving the compressor is a heated combination (typically about 165 to 195 degrees F.) of ammonia gas and dispersed oil droplets.
It is known to cool the ammonia stream in a heat exchanger wherein the heat is exhausted to either a liquid or gas medium. However, cooling of the lubricating oil has been more difficult and has required exhausting the heat to a liquid heat exchange medium in order to cool the oil sufficiently while limiting the heat exchanger to an acceptable size.
Use of a liquid exchange medium such as water to cool the oil in an oil cooling heat exchanger is, for example, known, but requires that water be available at the use site. It also suggests the use of liquid tight pipes or other transport means in order to contain the water. If the water is to be reused, a further heat exchange process is required in conditioning the water for re-use. If the water is not to be re-used, water disposal should be planned. Also, in locations where temperatures below 32 degrees F. can occur, some provision must be made to avoid freezing of the liquid in the heat exchanger. Accordingly, use of water to cool the oil presents certain costs associated with acquiring the water, controlling the water, protecting the water from freezing, and disposing of the water and/or its absorbed heat.
It is known to circulate a fraction of the liquified refrigerant to an oil cooler to cool the oil and thereby gasify the refrigerant, which is then circulated back to the condenser for condensing. That obviates the water requirement. But the net effect is to increase the heat exchange demand on the refrigerant condenser.
Any such secondary heat transfer in the system, whether to, for example, water or refrigerant, thus presents its own inefficiencies and entropy losses.
Just as small refrigeration assemblies (100,000 Btu/hr or less) requiring only electrical utilities, have enjoyed substantial commercial success, it would be desirable to have larger capacity refrigeration assemblies (greater than 100,000 Btu/hour) which have similarly minimal requirements of externally-provided utilities, namely only motive power utilities; and are truck transportable, as assemblies, to their work sites. This would provide the efficiencies and quality of factory assembly to larger refrigeration systems. Accordingly, cost, quality, and consistency of product could thereby be improved. To the extent the system could be made compatible with refrigerants more friendly to the environment than chlorofluorocarbon refrigerants, the potential threat to the environment can be controlled. To the extent inexpensive refrigerants can be used, cost can be contained.
It is an object of this invention to provide improved refrigeration units wherein lubricating oil is intermingled with the refrigerant in the compressor, and wherein the lubricating oil discharges its heat directly to the ambient air through a novel oil-to-air heat exchanger.
It is a special object to provide such a refrigeration system wherein ammonia is used as the refrigerant and wherein the heat discharged from the oil is sufficient to control the outlet temperature of the compressor at a temperature compatible with long term stability of the system, and especially compatible with long use life of the compressor.
It is a further object to provide such a system which is both truck transportable at standard cargo dimensions and weight, and which has a heat exchange capacity to ambient air of at least 200,000 Btu/hour at 95 degrees F. ambient air temperature.
It is another object to provide a refrigeration system, with a subcooling subsystem in which the differential temperature of the refrigerant liquid between the inlet and outlet is substantially constant.
It is still another object to provide a refrigeration system with control valves adapted to trap refrigerant in the heat source heat exchanger which receives circulation of the external medium being cooled, and thus the heat being received into the refrigeration system.